The marking of plastics by laser and the welding of plastics parts by means of laser energy are known per se. Both are brought about as a result of absorption of the laser energy in the plastics material, either directly, by interaction with the polymer, or indirectly, with a laser-sensitive agent added to the plastics material. The laser-sensitive agent may be an organic dye or a pigment which, by absorbing the laser energy, produces a visible local change in color in the plastic. It may also be a compound which, when irradiated with laser light, is converted from an invisible, colorless form into a visible form. In the case of laser welding, the heating of the plastics materials as a result of absorption of the laser energy in the joining region is so great that the material melts and the two parts weld to one another.
The marking of production goods is becoming increasingly important in virtually all branches of industry as part of the general rationalization measures. Thus, for instance, there is a need to apply production data, batch numbers, expiration dates, product IDs, bar codes, corporate logos, etc. Set against conventional marking techniques such as printing, embossing, stamping, and labeling, laser marking is significantly quicker, on account of its contactless operation, and is more precise and is also unproblematic to apply to nonplanar surfaces. Since the laser markings are generated beneath the surface in the material, they are durable, robust and substantially more secure with respect to removal, alteration or even counterfeiting. Contact with other media, such as in the case of containers for liquids, and closures, is likewise uncritical for this reason—on the assumption, of course, that the plastics matrix is stable. Security and durability of production IDs, and also freedom from contamination, are extremely important in the case, for instance, of packaging of pharmaceuticals, comestibles, and beverages.
Laser marking technology has proven very suitable in particular in connection with the marking of plastics. To be able to carry out efficient marking of plastics it is necessary to generate sufficient interaction between the plastic to be marked and the laser light. Here, firstly, it must be borne in mind that the energy introduced into the plastics should not be too high, since it may destroy the plastics article or its texture. Secondly, the laser beam must not pass through the plastic without significant interaction, since in that case the plastic cannot be marked.
In order to increase the interaction of the laser beam with the plastic, plastics are used in which absorption agents, also referred to as absorbers, have been incorporated. These absorbers may be, for example, laser-markable polymers or else pearlescent pigments and metallic effect pigments.
In the case of pearlescent pigments and metallic effect pigments, the irradiation of laser light results in heating of these pigments. In the immediate environment of the pearlescent pigments and of the metallic effect pigments, there is then a thermal change in the plastic, e.g., a carbonization or a foaming of the plastic, thereby allowing marking or identification of the plastics article.
DE 197 26 136 A1 discloses the use of laser-markable polymers in the form of microground particles having a size of 0.1 to 100 μm. A disadvantage of these laser-markable polymers is that they can melt during the processing of the plastics doped with the laser-markable polymers. It is therefore necessary for the melting ranges of the incorporated laser-markable polymer and of the plastics system used to be harmonized with one another.
DE 198 10 952 A1 discloses the use of pearlescent pigments or metallic luster pigments as absorbers in plastics.
WO 2004/045857 A2 describes a laser marking process using an ink, possibly comprising pearlescent pigments and metallic effect pigments as laser-sensitive materials.
From WO 2007/062785 A1 is a laser-transferable security feature for which laser-sensitive materials can be used that include pearlescent pigments or metallic effect pigments or metal powders, including mixtures of these. Pearlescent pigments are particularly preferred here.
A disadvantage of the use of pearlescent pigments or metallic luster pigments or metallic effect pigments is that satisfactory contrast after laser marking is achievable only with a high level of pigmentation. The level of concentration that must be chosen for the quantity of pigment is so high that it automatically entails coloring of the plastic with the pearlescent or metallic effect pigments.
Accordingly, with the exclusive use of pearlescent pigments and/or metallic effect pigments, it is not possible satisfactorily to achieve high-contrast laser marking without appreciable coloring in the case of pearlescent pigments (pearl luster effect) or without appreciable metallic coloration in the case of metallic luster or metallic effect pigments.
Moreover, the plateletlike structure of the pearlescent pigments or of the metallic luster or metallic effect pigments, at the concentrations that need to be used, has the deleterious effect that the pigments, owing to their platelet structure, undergo orientation in the laminar flow that is an inevitable corollary of the operation of injection molding of the plastics mass, and this leads to flow lines or streaks in the plastics article produced.
To obtain a desired contrast in the laser marking of plastics, EP 1 145 864 A1 teaches using a mixture of metal powder and/or semimetal powder and of an effect pigment or two or more effect pigments based on phyllosilicates. Here again there is visible coloration of the plastic, which is unwanted for clear and transparent plastics. Moreover, the pearlescent pigments likewise deleteriously produce streaks or flow lines in the plastics articles produced.
DE 10 2004 053 376 A1 discloses colored laser markings and laser inscriptions on plastics that are based on the welding of a polymeric inscription medium to the plastics surface. The energy absorbers suitable for marking that are mentioned in this specification include spherical metal powders, among other absorbers. No details, however, are given concerning the size of the metal powders.
Among the absorbers for masking laser beams that are described in JP 11070734 AA are metal powders. In this case the metal powders are used in a concentration of 5% to 90%, based on the shielding layer. At these very high concentrations, hazing of the medium occurs inevitably.
In accordance with the teaching of DE 10 2004 045 305 A1, the problem that exists in the prior art of the absorbers sustainedly coloring the plastics to be inscribed can be eliminated by incorporating a boride compound, preferably lanthanum hexaboride, into the plastics material. A disadvantage is that these boride compounds, especially lanthanum hexaboride constitute a significant cost factor. Consequently these boride compounds are not suitable as a laser marking agent for widespread use.
In order to allow marking of transparent plastics materials without coloration, use is made, in accordance with the teaching of U.S. Pat. No. 6,693,657 B2 and also of WO 2005/047009, of a laser marking agent which comprises a mixture of antimony oxide and tin oxide. WO 2005/084956 describes high-transparency plastics materials which are laser-markable and/or laser-weldable by means of nanoscale indium-tin oxide or antimony tin oxide particles. A disadvantage is that, like any other antimony compound, antimony oxide is highly toxic. Consequently this laser marking agent poses, on the one hand, a considerable risk to environment and people, both in production and processing and in disposal, since, firstly, antimony or compounds containing antimony must be used and, lastly, the plastics articles which contain antimony and/or compounds containing antimony must be disposed of again.
WO 2002/055287 A1 describes a process for producing laser-welded composite moldings. It mentions metal flakes and metal powders as fillers. These fillers, however, are used at relatively high concentrations of 1% to 60% by weight, based on the plastics molding.